[Bug c++/98474] incorrect results using __uint128_t

["jeffhurchalla at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98474

--- Comment #1 from Jeff Hurchalla <jeffhurchalla at gmail dot com> ---
If I change only the optimization level in the compile command to -O1, then
echo prints the correct result 0.

I have a non-minimal (but still very small) test file on godbolt at
https://godbolt.org/z/4oqnYn
The godbolt link goes to two separate tests which produce the incorrect results
- you can toggle between them with the "#if 1".

There are two interesting things about the godbolt tests:
1. I get correct results when I choose "x86_64 gcc 4.9.3" and incorrect results
when I choose "x86_64 gcc 5.1".  The few earlier versions than 4.9.3 that I
tried were correct also, and the few later versions than 5.1 (including trunk)
that I tried were incorrect also.
2. Using "x86_64 gcc 10.2" (and I'd suspect any version >= 5.1), the tests
print to stdout
"inside 'if'"
but they do not print
"inside 'while'"
This should be impossible.